Add copy thread link action

[brsbl]

Human comments

None.

What was wrong

Thread action menus did not provide a reliable way to copy the current thread URL. The first implementation wired the URL only from the thread header, which allowed the header and sidebar … menus to drift.

What changed

• Adds Copy thread link to the shared thread-action item set used by the header, sidebar dropdown, desktop context menu, and compact long-press menu.
• Derives the link from the active app origin and canonical project/thread route inside the shared menu, so every instance follows the current environment while excluding transient panel, split, and query state.
• Adds focused coverage that the shared menu copies its canonical thread URL with success and failure feedback.

Screenshots

The comparison uses the same seeded thread, route, open-menu state, and 1440×1000 viewport, cropped identically to the header action area. The after evidence is from branch commit 8f0eef326, which contains the final production code; final head 1b911180c changes only the test interaction event.

Before — merge base 720b3163d	After — final production code 8f0eef326

Sidebar parity on the same production-code revision:

How you verified

• Chrome for Testing 151.0.7922.71 against the branch web app at 1440×1000.
• Hard reloaded the seeded thread, opened both header and sidebar … menus, copied from each, verified the exact clipboard URL, and observed success feedback.
• Confirmed no console errors during the changed flow.
• CI: all required build, typecheck, lint, app, server, integration, package, and packaging smoke checks pass on final head 1b911180c.
• git diff --check origin/main...HEAD
• Local CI-equivalent checks were not run per repository policy; PR CI is the validation path.

BB-Thread-ID: thr_dwsizgdy3q

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